The division cycle of eukaryotic cells is regulated by a family of protein kinases known as cyclin-dependent kinases (CDKs). The sequential activation of individual members of this family and their consequent phosphorylation of critical substrates promotes orderly progression through the cell cycle. CDK activities are regulated by cyclin binding, by both positive and negative regulatory phosphorylations and by polypeptide CDK inhibitors. Cellular differentiation is accompanied by the down-regulation of CDK activity, which occurs through at least two mechanisms: down-regulation of cyclin expression which is required for CDK activity; and induction of CDK inhibitor expression. CDK4 is a major catalytic subunit of mammalian D-type cyclins, which act during the G1 phase of the cell cycle to enforce the decision of cells to enter the S phase. The deregulation of eukaryotic CDKs is correlated with many types of cancers.
Three D-type cyclins (D1, D2 and D3) are differentially expressed in proliferating cells in response to various growth factor mediated signals. The D-type cyclins interact combinatorially with CDKs 2, 4, 5 and 6 to form active holoenzymes that facilitate progression through the G1 phase of the cell cycle into S phase. Expression of the D-type cyclins depends on mitogenic stimulation, regardless of the position of the cell in the cycle. Growth factor withdrawal leads to rapid cyclin D destruction, with an associated loss of CDK activity.
A classical example of terminal differentiation is that of skeletal muscle cells. Skeletal muscle differentiation entails the coordination of muscle specific gene expression and terminal withdrawal from the cell cycle. Muscle differentiation is intimately coupled to the cell cycle, such that muscle-specific transcription is initiated only when myoblasts are growth arrested in the G1/G0 phase.
MyoD is a basic helix-loop-helix (bHLH) protein which plays an important role in the differentiation of skeletal muscle by inducing muscle structural gene expression and cell cycle withdrawal. In particular, MyoD is one of several transcriptional activators of muscle specific gene expression. The mechanisms by which MyoD induces myogenesis involve both the activation of muscle-specific gene expression and withdrawal from the cell cycle. Although proliferating myoblasts express MyoD throughout the cell cycle, the functions of MyoD are repressed in proliferating myoblasts.
Tumor suppressor retinoblastoma protein (RB) suppresses cell proliferation and thus plays an important role in the production and maintenance of the terminally differentiated phenotype of muscle cells. RB also appears to participate in control of entry into the S phase of the cell cycle. Inactivation of RB in terminally differentiated cells allows the cells to reenter the cell cycle. Correspondingly, loss of a functional RB is key step in the development of many human tumors.
The activity of RB is modulated by a phosphorylation/dephosphorylation mechanism during cell proliferation and differentiation. In resting or differentiated cells, RB protein is present in its dephosphorylated from. Underphosphorylated RB inhibits growth promoting transcription factors in the E2F/DP family. In rapidly proliferating cells, RB protein is highly phosphorylated. Maximal phosphorylation is associated with S phase of the cell cycle. Cyclins and CDKs contribute to the sequential phosphorylation of RB to inactivate its growth suppressive function.